-Achievements and limitations of classical mechanics

This paper discusses the achievements of classical mechanics, criticizes the viewpoint that the absolute time, absolute space, gravity essence and mass of classical mechanics remain unchanged, and explains its scope of application and its contradiction with classical physics.

Keywords: the contradiction between the essential quality, speed and energy of space-time gravity

First, the achievements of classical mechanics

The theoretical system of classical mechanics is based on Newton's three laws of motion. Newton systematically summarized the work of Galileo, Kepler and Huygens, and obtained the law of universal gravitation and Newton's three laws of motion, and published Mathematical Principles of Natural Philosophy in 1687. This is Newton's masterpiece and a classic of mechanics. In this book, Newton started with the basic concepts of mechanics (mass, momentum, inertia, force, etc.). ) and the basic laws (three laws of motion), and using the sharp mathematical tool of calculus invented by him, a complete and rigorous classical mechanical system was established, which unified the celestial mechanics with the ground object mechanics. This is the first big synthesis in the history of physics. Therefore, the publication of Newton's Mathematical Principles of Natural Philosophy marks the establishment of a classical mechanical system. This has a far-reaching impact on the process of scientific development and the way of thinking of later scientists. The establishment of Newtonian mechanics marks the birth of modern theoretical natural science and has become a model of other natural sciences.

Second, the limitations of classical mechanics

People who make history are always inevitably restricted by history, and Newton is no exception. Due to the limitations of the times, the basic concepts and principles of classical mechanics founded by Newton have inherent limitations, mainly in the following aspects:

Firstly, the basic concepts such as absolute time and absolute space are introduced. According to Newton, absolute, real and mathematical time itself is disappearing, and because of its nature, it is disappearing evenly and independently of any other external things. Absolute space, by its nature, has nothing to do with any external things, and it will never change or move. Absolute motion is the movement of an object from one absolute position to another.

Leibniz, Becker and Mach all put forward valuable objections to the concept of absolute space-time, pointing out that there is no evidence that Newton's absolute space exists. Einstein popularized the above principle of relativity and put forward the special theory of relativity. In the special theory of relativity, length and time interval also become relative quantities. The moving ruler is shorter than the static ruler, and the moving clock is slower than the static clock. In general relativity, the nature of space-time is not independent of the motion of an object: on the one hand, the nature of the motion of an object depends on what kind of space-time reference system is used to describe it; On the other hand, the nature of space-time also depends on the object and its motion itself.

The development of quantum theory raises a more basic question about the concept of time. One conclusion of quantum theory is that the judgment result of what state a system may have existed in the past depends on how to choose in today's measurement. This relationship between the present and the past and the concept of causal sequence are very different. The causal order implied in the concept of time requires that the existence of the past should be independent of the present.

Therefore, it is not always appropriate to describe the order of events with time. Space and time are an order between things, but not necessarily the most basic order, it may be an approximation of a more basic order.

Secondly, although Newton was cautious about the nature of gravity, he finally summarized it in the form of abstract and pure mathematics and thought it was a direct and instantaneous force at a distance.

Einstein's general theory of relativity explains gravity that space-time itself is elastic and can bend and stretch. When a mass object is placed in a certain space, the space will bend and deform. The greater the mass, the more serious the spatial bending deformation. So, why does space bend around objects with mass? Einstein also failed to give an answer. So Einstein's theory of curved space does not explain the nature of gravity. The explanation of quantum mechanics about the principle of electromagnetic force transfer between charges and the strong interaction between hadrons does not explain the nature of gravity. The view that gravity is transmitted by gravitational field or graviton has not been affirmed, because scientists have not found a graviton that transmits the gravitational effect so far.

Thirdly, in classical mechanics, the mass of an object is constant and has nothing to do with its speed or energy.

In the theory of relativity, the extension of the concept of mass has been greatly expanded. Einstein's famous mass-energy equation E=mc2 combines the independent laws of conservation of mass and energy in classical mechanics into a unified law of conservation of mass and energy, which fully embodies the unity of matter and motion. The mass-energy equation shows that mass and energy are inseparable and related. On the one hand, any material system can be marked by both mass m and energy e; On the other hand, when the energy of one system decreases, its mass also decreases, and when another system receives and increases energy, its mass also increases accordingly.

Einstein considered the elastic collision of two spheres from the mechanical point of view, and derived the famous formula of mass velocity by using the theorem of conservation of momentum and the theorem of relativistic velocity addition.

This formula shows that the mass of an object is no longer a quantity independent of its motion state, but depends on its motion speed. When the velocity of a moving object is v, the mass is, where m0 is the static mass of the object. When the speed of an object tends to the speed of light, the mass of the object tends to infinity.

Fourthly, the classical laws of mechanics are only applicable to the macroscopic low-speed world, and the high-speed situation equivalent to the speed of light and the application of the microscopic world were not involved and could not be involved at that time.

Fifth, the potential contradiction between classical physics and classical mechanics.

In classical physics, the description of light is probably the most difficult to satisfy. If the particle theory is correct, then people can't help asking, when light is absorbed, what are the particles that make up light? Moreover, in order to represent both measurable matter and light, different entities must be introduced into the discussion, which is not reassuring in any case.

Similarly, the wave theory of light incorporated into the mechanical framework is also difficult to justify. According to wave theory, light is interpreted as the vibration of the ether that fills the universe. Because light is a shear wave, ether must have the ability to withstand shear stress rather than compressive stress, and because ether does not produce observable resistance to measurable substances, it must have a very small density. To this end, people have racked their brains and imagined various etheric models. This omnipotent wonderful ether is actually confusing.

The basic concepts and principles of classical mechanics have also encountered some troubles in thermodynamics. 1865, clausius established the second law of thermodynamics, revealing the irreversibility of physical processes related to thermal phenomena. In classical mechanics, no similar situation has ever been found, and the reversibility of mechanical process is guaranteed by universal mechanical principles. But the second law of thermodynamics is also universally established, so this contradiction cannot be explained by the basic concepts of mechanics.

Third, summary.

Newton spent his whole life building a scientific monument. His research has promoted the progress of human civilization, and his achievements in various aspects of macroscopic physics are extremely extensive and brilliant. However, people who make history are always inevitably restricted by history, and Newton is no exception. Due to the limitation of the times, Newton denied the wrong exposition and fuzzy concepts since Aristotle and founded Newtonian mechanics, but it also implied the potential factors of self-denial. As Engels said: "Everything that is realistic in the field of human history will become unreasonable with the passage of time; Therefore, it is unreasonable in essence, and it has been unreasonable from the beginning. " (Selected Works of Marx and Engels, Volume IV)

Because Newton tried his best to show that his system was determined by the inevitability of experience, especially because of the great achievements of classical mechanics in practice, it was enough to hinder future generations from thinking about the transcendental characteristics of those basic concepts and principles, so that no one thought that the whole foundation of physics might need long-term fundamental reform. In fact, physics has its own basic concepts and principles in each historical period, and in the following period, people tend to exaggerate its role and misuse it improperly, beyond its reach. In order to eliminate this misuse, every historical period needs a new enlightenment, and it is this endless spirit of enlightenment that keeps science from becoming a rigid dogma.

References:

[1] Zhang Qiren's Classical Field Theory, Beijing: Science Press, 2003.

[2] Quantum Mechanics Jing Harbin: Harbin Institute of Technology Press, 2004

[3] Space: From Relativity to the History of M Theory. Guan: Tsinghua University Publishing House, 2004.

[4] Time Paul? 6? Bennett's1; Shanghai: Shanghai People's Fine Arts Publishing House, 2003.

[5] special theory of relativity G. Stevenson; Shanghai: Shanghai Science and Technology Press, 1963.

[6] Guide to Relativity Zhao Zhanyue Beijing: Tsinghua University Publishing House, 2002.

[7] Thermodynamics by Wang Zhuxi: Peking University Publishing House, 2005.

[8], edited by Shen, History of Physics, Tsinghua University Publishing House, 1993.

[9] College Physics. Editor zhong. Beijing: Higher Education Press, 2004.